198 HAWAIIAN AND OTHER PACIFIC ECHINI. 



assume bizarre forms, either of stalk or of valves. Occasionally there may 

 be only two valves and in rare cases there may be four, five, six, or even eight 

 valves, but such pedicellariae seldom throw much light on intergeneric relation- 

 ships. 



While recognizing no fewer than twenty genera, it must be admitted that 

 they are of unequal value and by no means always easy to distinguish. Yet 

 the future is likely to see more rather than fewer genera recognized, even if 

 no new species are discovered. Those genera which possess an internal fasciole 

 are probably the most highly specialized sea-urchins now existent, unless we 

 except Pourtalesia and its allies. The absence of a peripetalous fasciole in 

 seven of the genera seems an unquestionably secondary character and not 

 the primitive condition. 



Key to the Genera of Spatangidae. 



No internal fasciole. 



Peripetalous fasciole present, at least in part. 



Petals well formed, and in adults depressed, often flush in young specimens. 

 Peripetalous fasciole a single undivided band. 



Posterior end of test not projecting as a subanal snout encircled 

 by the subanal fasciole. 



Ambulacrum III dorsally more or less distinctly sunken and 



petaloid; posterior petals often the shortest BrLssopsia. 



Ambulacrum III little or not at all sunken and not at all peta- 

 loid; posterior petals usually the longest. 



A distinct anal fasciole arises from subanal on each side of 

 the periproct. 

 Ambulacrum III more or less differentiated abacti- 

 nally, often depressed; subanal plastron not pro- 

 jecting. 



Big primary tubercles present within peripeta- 

 lous fasciole in interambulacra 1 and 4, and 



may be in 2, 3 and 5 Plagiobrissus. 



No big primary tubercles within peripetalous 



fasciole Metalia. 



Ambulacrum III not at all differentiated abactinally. 

 perfectly flush with adjoining interambulacra; 

 subanal plastron projecting downward or back- 

 ward more or less distinctly Rhynobrissus. 



No anal fasciole. 



Subanal fasciole complete, surrounding a distinct 



subanal plastron Brissu*. 



Subanal fasciole incomplete; no distinct subanal 



plastron Meoma. 



Posterior end of test projecting as a subanal snout encircled by the 

 SUbamJ fasciole Cionobrissu*. 



